The invention relates to optical devices. More particularly, the invention relates to an interleaving comb filter that can be used, for example as an interleaver and/or a deinterleaver having a feedback loop to reduce chromatic dispersion.
Development of future optical fiber communications system can be characterized by three trends: higher speed, better spectrum efficiency (i.e., closer channel spacing) and longer transmission reach. To achieve these goals, passive dense wavelength division multiplexing (DWDM) have been developed, which multiplex signals on a single optical fiber using different frequencies, or channels. In DWDM system components, it is generally desirable to have the following characteristics: low dispersion, small channel spacing, wide flat-top passband, low; insertion loss and high isolation.
DWDM systems use multiplexers and demultiplexers to operate on optical signals having multiple channels. Interleaving and deinterleaving of optical channels allows a subset of channels to be filtered from or added to another set of channels. Interleavers and deinterleavers are typically manufactured using various combinations of crystals and/or other components. The closer the channels become in terms of frequency, the more important the effects of chromatic dispersion become. Chromatic dispersion is wavelength dependent speed of propagation and causes a signal to spread or disperse.
Chromatic dispersion in an optical device normally has two origins: material dispersion and structural dispersion. In most interleaver/deinterleavers in the prior art, chromatic dispersion is dominated by structural dispersion because the length of the material (e.g., fiber, glass, crystal) used to construct the interleaver/deinterleaver is relatively short. Thus, the structure of the interleaver/deinterleaver can increase or decrease the chromatic dispersion based on the design used.
Traditional techniques for reducing chromatic dispersion include matching components having equal and opposite dispersion characteristics such that the dispersion from one component cancels the dispersion from another component. Such matching techniques are complex and difficult, which results in an increased cost and complexity of the optical device. Moreover, these techniques may not provide sufficient reduction of chromatic dispersion for channels having a spacing of 25 GHz or less.
Optical filters and optical interleaver/deinterleavers are described. In one embodiment, the optical filter includes two optical fibers, each having a first end and a second end. The first and the second fibers are fused together in multiple places having, each of the fused sections having coupling ratios such that an optical signal having multiple frequencies input to the first end of the first fiber is filtered into a first filtered signal including a first subset of the multiple frequencies at the second end of the first fiber and a second signal including a second subset of the multiple frequencies at the second end of the second fiber, the first signal having a chromatic dispersion caused by passing the first signal from the first end of the first fiber to the second end of the first fiber. A non-reciprocal optical element, for example, an optical isolator, is coupled between the second end of the first fiber and the second end of the second fiber, the non-reciprocal optical element passes the first signal from the second end of the first fiber to the second end of the second fiber. The passing of the first signal to the first end of the second fiber reduces the chromatic dispersion caused by passing the first signal from the first end of the first fiber to the second end of the first fiber.